I've got the annoying p0401 code. I just replaced the DPFE and the light came back on so I swaped with another brand new DPFE, code came back.
Right to my question. How can I tell if my EGR valve is stuck closed? That would set off the the insufficient flow right?

 

I have had the same problem about every 30-50,000 miles. Found this video that explains the symptoms and fixes very well. YouTube - ‪Wells Ford EGR DPFE Training with Sgt. Tech P0400, P0401, P0402,‬‎
Hope it helps!

 

Pull a vacuum on the EGR valve with the engine at idle. When the vacuum is applied the engine should start to run rough. If it doesn't, the valve is stuck closed. You can also verify the solenoid operation by jumping 12 volts into it. If the engine sputters (runs rough) you have verified both the solenoid and valve are good.

 

check your hoses that go to the dpfe, i have seen one that had a hole in it.

 

I forgot to post what was wrong, I started another thread about it also, but after throwing all new parts, egr, vacum syl, and two new dpfe's at it and removing the whole tube and soaking in simple green I still had the same problem. The fix you ask? A third new dpfe. The first two brand new sensors were faulty. After much inspection I realized the top of the sensors didn't appear to be sealed so I ran off my hunch and was right.